Algae build-up in storage tank

[PurewaterGuy]

I'm experiencing green algae in a storage tank. This is the only system I have that has this problem. Located 40 minutes south of Boston. System is set-up as follows.

(1)- 12"x52" Turbidity Unit, (2) - 12"x65" Carbon units (6cft) (1)- 20" prefilter 5 micron, (1)- Reverse osnmosis Unit with (2) 4"x25" Thin film membranes, 2.5 GPM Permeate.

The rejection rate is 90%

RO feeds a 200 Gallon Fiberglass Storage Tank.

In March I did a complete system overhaul, rebeds, membrane change out and storage tank cleaning. The tank showed signs of algae in March. Never happened in the previous 13 yrs.

The storage tank is connected to a distribution pump. Water usage on a 6 day 24hr cycle is 385-390 Gallons per day. The only day the water sits stagnant in the tank is Sunday.

The water leaves the distribution tank and goes through (1) 3.6cft bed of carbon and (2) 3.6cft mixed-bed deionizers, (1) Ultraviolet unit, (1) 0.2 micron postfilter. The water does not recirculate back to the storage tank, it is a closed loop system.

What I propose to do. Add (1) Ultraviolet unit post Carbon pretreatment prior to RO ,,, or add (1) small recirculating pump to keep the water in the carbon beds moving (although I don't beleive channeling is a factor)or, Have the loop water recirculate back to the storage tank.

Any thoughts?

The feed water does contain chlorine from the town.

 

[bimr]

Ultraviolet equipment provides an economical means of water disinfection for water manufacturing facilities.

The application provides disinfection from the feed water source to the storage tank. The water can be re-circulated from the storage tank back through the UV equipment in order to maintain bacteria reduction.

 

[PurewaterGuy]

I have ultraviolet in the distribution loop so I know I'm good there. I'm getting algae growth in the storage tank directly from the RO. I'm noticing green stains on the brass fittings of the pretreatment which would indicate Low PH. BUT, the algae growth would make one think the PH is high. Does that sound right?

I'll have to do some water testing.

 

[stanier]

Could it be the fiberglass allows light to penetrate and promote the growth of algae? Painting the tank may be a solution. In desalination plants this is done to prevent algae growth.

Also there may be some water permeation into the FRP and this harbours algae spores. To thoroughly disinfect may require a higher dose of sodium hypochlorite and holding in the tank for 48 hours or more.

 

[bimr]

Do you have the correct type of tank to prevent algae?

Opaque tanks will prevent the algae from growing:

http://www.amprotec.net/water_tanks_1.htm

 

[PurewaterGuy]

The tank in place is in fact fiberglass. I know not the best choice by far. The tank has been in place from 1990 until now. There is a constant light source right above the tank, flourescent lights. This was pointed out to me before, just after I started to experience this problem. Strange it would wait 18 years before it starts this though? I suppose it wouldn't hurt to take the tank out of line and hit it with a 35% solution of Hydrogen Peroxide, rinse, test and put back online. Refill tank and hit it with an ozonation. Then cover the exposed tank or paint it.

I was considering returning the loop water back to the tank, but after thinking on it I really don't like to do that. It seems to me it would be a waste of DI water and make the DI tanks work harder. Also, I have heard in the past that DI water is not the best thing to expose to fiberglass storage tanks in the long run - too aggressive - From your experience have you heard or seen any truth to this?

 

[bimr]

FRP is just a generic term. An FRP tank usually has a inner lining/coating that provides the actual chemical resistance. Ask to see the tank data sheet to determine what the "FRP" material actually consists of.

 

[PurewaterGuy]

All the storage tanks I use now to store RO water are HDPE

I never liked the fiberglass tanks to begin with. They're bulky and a PITA to move around.

The FRP tanks I use for carbon and DI exchange tanks are FRP. All Park international tanks, maybe a couple of less known brand names tanks in the inventory as well. As for the fiberglass storage tanks, I couldn't tell you, if there is in fact a liner of some sort it's not noticeable, maybe it's a spray on application invisible to the naked eye?

I use Baliff for all my tank products now. Great people, great product at a great price, at least for me. JMO

 

[PurewaterGuy]

Ok, been a while for this one. I did a cleaning of the fiberglass tank today. Definitely had green algae in it. I scrubbed it down and hit it with chlorinated water. I refilled the tank with RO water, RO water quality % of Rjection across the membranes was 95.7%. The meter on the RO shows it is making water quality of 1 meg. I treated the full storage tank with a good doseage of Ozone. I pushed the ozonated water throughout the building pipe loop as well.

I took a PH reading from the Permeate line feeding the storage tank and the test showed the water to very acidic, and I mean very acidic.

I'm just a bit confused here. If the water feeding the storage tank is very acidic, how is it possible for the tank to grow green algae?

Also, Would the water being acidic be an indicator that the carbon pretreatment prior to the RO has a exhausted bed?

What do you guys think?

 

[bimr]

"Also, Would the water being acidic be an indicator that the carbon pretreatment prior to the RO has a exhausted bed?"

No.

The water is acidic in the RO permeate because CO2 passes through the membrane and alkalinity is rejected by the membrane. The change ratio of alkalinity:CO2 after the RO unit is what causes the pH to change.

The CO2 is probably being stripped out in the tank, allowing a higher pH to occur in the tank than in the permeate.

 

[PurewaterGuy]

OK, I'm a little confused maybe. How would I get the higher PH in the storage tank if the water is acidic? I don't understand what you mean.

Another question.

With the water from the permeate being acidic going into the storage tank, would this have an adverse effect on the mixed bed deionizers I have in line after the storage tank? I also have a carbon tank in line after the storgae tank prior to the mixed bed deionizers, would this have an adverse effect on that?

Thanks, you're helping me here.

 

[bimr]

pH is a function of the alkalinity/CO2 ratio.

If you have a water with 200 mg/l alkalinity, at a pH of 6, the CO2 in the water is 100 mg/l.

The RO rejects 90+% of the alkalinity and 0% of the CO2.

Therefore, after the RO, your alkalinity/CO2 ratio is now 20 mg/l/100 mg/l = 0.2 which equates to a pH of 5.6 (in the permeate.

CO2 was commonly air stripped after the RO or after a cation unit. When CO2 is air stripped, the pH will rise. That maybe what is happening in your tank.

Low pH of permeate due to CO2, CO2 is air stripping out in the tank, resulting in an increase in pH in the tank.


The low pH of the permeate is not going to have much of an effect on the MB unit or the carbon filter.

 

[PurewaterGuy]

OK. So the one thing I didn't do was test the PH of the storage tank which I will now do.

If I find the PH in the tank to be high, what can I do to correct that situation?

 

[quark]

It can be acidophilic algae and CO[sub]2[/sub] favours the growth situation.

You didn't give any description of strippers in your scheme nor any dosing activities that increase pH of feed water to RO. In the absence of these activities, the likelyhood of increasing pH in the permeate water is nill. If you find the tank water to be acidic, then the algae can be acidophilic and you can control this by maintaining neutral pH. You can reduce the CO[sub]2[/sub] levels by maintaining feed water pH above 8.3 and thus converting it to corbonate, which can be rejected by RO.

Pleases note that CO[sub]2[/sub] increases ionic loading on the ion exchange system and this may effect your MB process.

 

[PurewaterGuy]

"You didn't give any description of strippers in your scheme nor any dosing activities that increase pH of feed water to RO."

By strippers are you refering to pretreatment?

Prior to the RO I have one 12"x52" Turbity filter followed by a 14"x65" Carbon Unit followed by a 20" 5 micron filter.

Could it be a problem with the pretreatment? If it could, what could the possibilities be? I guess what I mean is, Since the water coming from the RO permeate is acidic, is there something that could be done on the pretreatment side to stop this?

I have also had the lights above the storage tank turned off as suggested.

 

[PurewaterGuy]

See the post above before reading this one please, I don't see an edit feature for this site.

I have PH readings being taken from the feed source after the Pretreatment Unit and from the storage tank.

Will this give us a clearer picture of what is happening?

 

[bimr]

"Since the water coming from the RO permeate is acidic, is there something that could be done on the pretreatment side to stop this?"

Not really.

Many RO systems use acid pretreatment prior to the RO to lower the pH to prevent scaling internally in the RO. Unless the water is low in TDS to start with, raising the pH to reject allow more alkalinity to be rejected (increasing the permeate pH) may be counterproductive because of the increased scaling potential of higher pH water.

"You didn't give any description of strippers in your scheme nor any dosing activities that increase pH of feed water to RO."

Removing the CO2 is a post RO treatment step. You can use a air stipping device to remove CO2 or a membrane device. The membrane device is similar to the RO unit

 

[PurewaterGuy]

I'm getting a real education here and I really appreciate this. I'm learning more in this one posting than I have about anything in the water industry in years. It's good to have to think again.

Would it be possible to remove the CO2 by way of aeration?

If it is, I could re-route the pipe loop back to the storage tank. As it stands now the loop is a closed loop and water is drawn into the loop as demand is needed. The water does stand for a period of 36 hrs on the weekend.

 

[quark]

I worked on RO units with 1000 ppm TDS input water quarlity and we maintained high pH to take care of CO2. The scaling issue was tackled by dosing antiscalent prior to the RO.

Strippers or membranes are another option to remove CO2. If the microbial load is of concern then strippers should use 0.22 micron filtration for the incoming air.

 

[bimr]

You have to know how much CO2 you have in the first place before doing anything else.

Everything starts with water analysis.

Most pure water applications do not use strippers because the strippers will contaminant the water with airborne contaminants.

If you do have significant CO2 (maybe higher than 50 mg/l), you should investigate a membrane separation process. Otherwise, you MB unit will remove the CO2.